There are conventionally known structures having voids obtained by consecutively folding and laminating a film so as to form the film into a specific three-dimensional structure. There are also known electret filters using the principle of electrifying (electretizing) the film of such a structure and adsorbing dust with an electrostatic force when air containing dust is circulated into the voids thereof. For example, it is disclosed in Patent Documents 1 and 2 that electret filters are obtained by consecutively folding an electretized film and laminating the film in the thickness direction.
These electret filters have the advantage that the pressure loss is low. However, since the amount of electrification of these electretized filters is low, the volume charge density of the space enclosed by the film is low, so it has not been possible to increase the dust-collecting capability. In addition, the dissipation of the amount of electric charge (electrification amount) accumulated in the film is fast, which has the drawback that a stable collecting capability over a long period of time cannot be achieved.
Further, it is disclosed in Patent Document 3 that an electret filter is obtained by imparting electret characteristics to the outside wall of an arrayed body having a channel enclosed by a plastic wall. However, when performing electrification treatment for imparting electret characteristics, treatment is performed by filling the outer surfaces of the sheet material with an electrically conductive liquid which connects the groove inner part to the ground potential and the outer surfaces of the sheet material to a high negative and positive potential, respectively, so an apparatus for electrification is required, and a high voltage is applied by this apparatus, which carries a risk of electric leakage or electric shock due to insulation breakdown.
In addition, Patent Documents 4 and 5 disclose filtration devices in the form of a structure having voids due to a film having a structure with a high aspect ratio structure for forming flow channels, wherein the film is electrified. Although these filtration devices have the advantage that the collection efficiency improves due to a high surface area, there are undulations on the film surface, so the workability is diminished when forming the film into a structure, and the space volume of the filter decreases, which has the drawback that the pressure loss increases. In addition, since there are no voids inside the film, the electrification dissipation rate is high, which has a risk that the sustainability as an electret filter may be diminished.
On the other hand, Patent Document 6 discloses a sheet formed by laminating (1) a film prepared by processing a film having a specific porosity and compression height into a wave shape having a wavelength of from 0.5 to 60 mm and a height of from 1 to 20 mm of the highest and lowest parts of the wave and (2) a thermoplastic resin film on at least one surface thereof; and a structure formed by laminating a plurality of these sheets. Such a structure has a possibility that the film surface may be electrified by friction associated with the circulation of air, but even when electrified, the function as a filter is insufficient since the charge density of the film surface is small.
In addition, Patent Document 7 discloses a filter with a trapezoidal corrugated shape comprising a first surface electrified positively and a second surface electrified negatively opposite the first surface. However, there is no specific description of a method of increasing the amount of electrification charge, and an electret filter obtained with the production method of this document has a high electrification dissipation rate, which has a risk that the sustainability as an electret filter may be diminished.